I posted my thoughts about why Aika remains in a round-about-way over on H!O (but it was a bit off-topic in that particular thread, so I kept it short).
Assuming any future graduates don't stay with H!P and go over to UF, we have this inevitable situation:
After Sayu graduates, Captain should theoretically become H!P leader.
After Shimizu grads (likely upon Berryz going as a whole), then Yajima would be H!P leader.
Once H!P Kids graduate, who will then take over the reigns?
Discount Sayu and H!P Kids, the oldest members of H!P would be as follows:
Mitsui Aika [1993.01.12]
Miyazaki Yuka [1994.04.02]
Wada Ayaka [1994.08.01]
Iikubo Haruna [1994.11.07]
Fukuda Kanon [1995.03.12]
Kanazawa Tomoko [1995.07.02]
Apart from Aika, everyone else on that list is either very new to H!P/broad leadership roles or will likely be graduating in the next few years (looking at you S/mileage first gen).
Mitsui has not only been with H!P 7 years, but (as the article posted earlier in this thread discusses) she also has considerable amounts of directly leading new members as well as experience in all the various media H!P is involved in (radio, tv, concerts, photobooks, plays). She knows what it takes to get the job done well and isn't afraid to make you hate her to realize how you need to improve to contribute. The most convenient thing? If the Kids groups leave before Sayu does, H!P leadership would remain in a former MM member's hands.
If anything, H!P makes long-term plans and investments and stick to their guns. This push toward an English-speaking audience and plans for more western promotion tactics aligns pretty conveniently with Aika going abroad to study English. She's a smart girl, so I don't doubt she'll do well. Image how awesome it would be to have a H!P concert in Europe or the Americas and Aika representing H!P as leader doing ENGLISH interviews to explain more about the label and girls. I could think of a dozen reasons why she would be the smartest choice to make this kind of investment in.
TLDR; Think of the long-term timeline and goals. She's proved she's got the tough skin and consistent hardwork/dedication for the job of being a senior/leader of H!P as the label grows and expands. Why wouldn't you keep her around? I also ask myself this of Sayu, since I want her around forever
.